Glaucoma refers to a series of relatively common eye disorders in which pressure within the eye is sufficiently high as to result in damage to sensitive intraocular structures, including the retina and optic nerve. Glaucomas are classified as primary (including chronic open angle glaucoma, angle closure glaucoma, mixed mechanism glaucoma and infantile glaucoma and secondary (related to other diseases of the eye). The elevation of intraocular pressure ultimately leads to irreversible destruction of the optic nerve. The clinical symptoms, which are not readily recognized in the early stages, are characterized mainly by a slow, relentless, progressive narrowing of the field of vision, and decrement in visual integration processing, including diminished dark adaptation. In the absence of treatment, the eventual outcome is total loss of vision often accompanied by severe eye pain.
In order to fully appreciate the described embodiments, a brief overview of the anatomy of the eye is provided. As schematically shown in FIG. 1, the outer layer of the eye includes a sclera 17 that serves as a supporting framework for the eye. The front of the sclera includes a cornea 15, a transparent tissue that enables light to enter the eye. An anterior chamber 7 is located between the cornea 15 and the crystalline lens 4. The anterior chamber 7 contains a constantly flowing clear fluid called aqueous humor 1. The crystalline lens 4 is connected to the eye by fiber zonules, which are connected to the cilliary body 3. In the anterior chamber 7, an iris 19 encircles the outer perimeter of the lens 4 and includes a pupil 5 at its center. The diameter of the pupil 5 controls the amount of light passing through the lens 4 to the retina 8. A posterior chamber 2 is located between the crystalline lens 4 and the retina 8.
As shown in FIG. 2, the anatomy of the eye also includes a trabecular meshwork 9, a narrow band of spongy tissue within the eye that encircles the iris 19. The trabecular meshwork (“TM”) varies in shape and is microscopic in size. It is generally triangular in cross-section, varying in thickness from about 100 μm to 200 μm. It is made up of different fibrous layers, having micro-sized pores forming fluid pathways for the egress of aqueous humor from the anterior chamber. The trabecular meshwork 9 has been measured to a thickness of about 100 μm at its anterior edge, Schwalbe's line, 18 at the approximate juncture of cornea 15 and sclera 17.
The trabecular meshwork widens to about 200 μm at its base where it and the iris 19 attach to the scleral spur. The passageway through the pores in the trabecular meshwork 9 lead through a very thin, porous tissue called the juxtacanalicular trabecular meshwork 13, which in turn abuts the interior side of a structure called Schlemm's canal 11. Schlemm's canal 11 is filled with a mixture of aqueous humor and blood components and branches off into collector channels 12 that drain the aqueous humor into the venous system. Because aqueous humor is continuously produced by the eye, any obstruction in the trabecular meshwork, the juxtacanalicular trabecular meshwork or Schlemm's canal, prevents the aqueous humor from readily escaping from the anterior chamber. This results in an elevation of intraocular pressure in the eye. Increased intraocular pressure can lead to damage of the optic nerve and eventual blindness.
Present surgical techniques to lower intraocular pressure include procedures enabling fluid to drain from within the eye to extra ocular sites. However, these drainage or “filtering” procedures not only increase the risk of causing a lens cataract, but often fail by virtue of their closure resulting from the healing of the very wound created for gaining access to the surgical site. Ab inferno surgical procedures, also, if not adequately stealth eventually fail. In creating the egress by photoablation or by photodisruption less inflammation at the egress site is induced than by current techniques, thus prolonging filtration wound function.
Lasers were first used in 1965 to repair retinal detachments. The procedure involved chorioretinal coagulation in which a laser beam positioned from without the eye was used to achieve fusion of the retina and the choroid. The technique consisted of introducing a laser beam from outside the cornea, and by employing the refractive media of the eye itself, the laser radiation was directed in such a manner that it was concentrated at a selected point upon the retina/choroid so that the tissues in a very localized area were photothermally congealed.
In contrast to thermal energy produced by visible and by infrared lasers, such as Nd:YAG systems, the high photon energies of femtosecond lasers can photodisrupt the material in question, namely eye tissue, in a manner which does not cause significant target tissue temperature elevation. By photodisruption, both visible and infrared femtosecond laser radiation can be used to drastically alter the target tissue in a “cold” environment. This becomes significant for controlled removal of organic substances, such as living tissue, in contradistinction to treatments in which heat is generated, e.g. by thermal lasers, which could damage, if not destroy, delicate eye tissue adjacent to the target sites to be removed and thereby induce healing responses.
Femtosecond (“FS”) lasers used for this purpose include the group of rapidly pulsed lasers which emit at 0.4 to 2.5 μm in the visible and infrared spectra. In contrast to the thermal visible and infrared radiation from some Nd:YAG or CO.sub.2 lasers or the like, the high energy photons from femtosecond lasers at photodisruptive fluence levels are absorbed by the target tissues. This absorption creates a hot plasma at the focus, vaporizing tissue. The plasma subsequently expands supersonically launching a pressure wave. Later, a cavitation bubble forms and eventually collapses. The extent of the tissue damage caused by the pressure wave and cavitation bubble expansion is energy-dependent. Femtosecond pulses deposit very little energy while still causing breakdown, therefore producing surgical photodisruption while minimizing collateral damage.
Juhasz T, Chai D, Chaudhary G, et al.; Application of Femtosecond Laser Surgery for the Treatment of Glaucoma; in Frontiers in Optics, OSA Technical Digest (CD) (Optical Society of America, 2008) disclosed that FS laser pulses could be used to create partial thickness scleral channels that drain aqueous humor into the sub-conjunctival space, showing potential for the treatment of glaucoma. Toyran S, Liu Y, Singha S., et al.;
Femtosecond laser photodisruption of human trabecular meshwork: an in vitro study; Exp. Eye Res. 2005; 81(3); 298-305, disclosed the use of FS lasers to perform photodisruption of human TM strips ex vivo, creating full-thickness ablation channels through the TM without collateral damage. The ideal settings for creating lesions with minimal collateral side effects on the inner surface of the TM are: Ti: Sapphire laser beam (45 fsec, 1 kHz, 800 nm) with 14.4 mJ pulse energy and an exposure time of 0.5 sec Nakamura H, Liu Y, Witt TE, et al.; Femtosecond laser photodisruption of primate trabecular meshwork: an ex vivo study; Invest. Ophthalmol. Vis. Sci. 2009; 1198-204 disclosed photodisruption by FS laser of the TM of ex vivo, intact, enucleated human and baboon eyes. The settings were 45 fsec, 1 kHz, 800 nm with 60 to 480 μJ and 0.001 to 0.3 sec exposure time. The study showed that laser ablation of the TM ab interno in ex vivo primate eyes is feasible by a custom femtoseeond laser ablation system with a gonioscopic lens. The photodisruption did not reach Schlemm's canal, although this goal could easily be achieved through an alteration in laser settings and delivery methods.
However, successful use of ultrashort laser pulses, such as those produced by a FS laser, in vivo to produce channels in the TM to relieve glaucoma has not been demonstrated by the work discussed above and presents challenges not present in those experimental studies. In particular, the laser beam must be delivered to a precise location on the TM in such a way as to avoid damage to adjacent and intervening tissue. The challenges of delivering adequate photodisruptive energy in precise patterns in shapes and depths include (a) the curved surface of the target, (b) the target lies beyond the critical angle of visible ocular structures seen through the cornea unaided, (c) optical coupling systems are necessary to visualize and target the intended treatment sites, e.g. trabecular meshwork and Schlemm's canal, (d) the location of Schlemm's canal may be difficult to establish particularly as it lies behind the optically significant trabecular meshwork, (e) once the inner wall of Schlemm's canal therefore the “blood aqueous bather”, is penetrated, blood components may obscure the optical pathway for any future optical viewing and/or treating beams.